Practical method of determining plagioclase twinning laws under the microscope.

Abstract

(2002) Practical method of determining plagioclase twinning laws under the microscope. Bull. Geol. Surv. Japan, vol. 53 (11/12) p.795-800, 5 figs., 1 table. Abstract: Correspondence between various rock types and frequency of plagioclase twinning laws must be revised with the recent geological background for improving the criteria to discriminate geological settings by plagioclase twinning laws. For this revision, a rapid and reliable method to determine plagioclase twinning law types is necessary. Twinning laws can be recognized in a thin section with the aid of a universal stage. The plagioclase twinning law is determined from the composition plane (CP) and position of the symmetry axis (SA). The CP is determined with cleavages or optical elongation at the diagonal position. Normal, parallel or complex twin types can be understood by optical characters based upon the relationship of SA and CP. Based upon combinations of these characters, we can distinguish most plagioclase twinning laws in natural rocks.